Is electricity a dimensionless quantity or not?

Common non-SI units

According to the SI (international system of units) units for physical quantities should preferably be used from this system of units. In addition, however, other units explained below are also in use.

Non-SI units accepted for use with the SI

Chapter 4 of the so-called SI brochure lists units that do not belong to the SI, but can be used together with it. The following examples of such units are arranged according to their respective size. Some of these units can also be used with SI prefixes.


1 ha = 1 hm2 = 104 m2

The (or the) hectare is in Germany and Switzerland - there with the unit name "hectare" - the legal unit for the indication of the area of ​​land and parcels, thus not a generally valid legal unit of area.


1 l = 1 L = 1 dm3 = 10−3 m3


1 t = 103 kg


  • Minute (min), hour (h) and day (d):
1 min = 60 s
1 h = 60 min = 3600 s
1 d = 24 h = 86 400 s

The use of SI prefixes with these three time units is not permitted according to the SI brochure. The year is not listed as an accepted unit in the SI brochure, but is from common usage - e.g. B. when specifying times for appointments and deadlines - indispensable.


1 bar = 100 kPa = 105 Pa
1 mmHg = (101 325/760) Pa ≈ 133.322 Pa (only for blood pressure measurement). In Switzerland and Germany, the unit with the unit symbol mmHg is the legal unit for specifying blood pressure and the pressure of other body fluids.

Dimensionless sizes

  • Degree (°), minute (′) and second (″) for the plane angle:
1 ° = (π / 180) rad
1 ′ = (1/60) ° = (π / 10 800) rad
1 ″ = (1/60) ′ = (π / 648 000) rad

Statutory non-SI units not accepted for use with the SI


1 a = 1 dam2 = 102 m2

The Ar is not listed as an accepted unit in the SI brochure, but in Germany and Austria as well as in Switzerland - there with the unit name "Are" - the legal unit for specifying the area of ​​land and parcels.

Not recommended but common non-SI units

In general, but also in technical usage of the German language, some units or unit symbols are (still) in use that are not or only partially compliant with the International System of Units. The following examples of such units, their symbols and names are arranged according to their respective size.


  • the inch (′ ′), e.g. B. to specify the diagonal of screens or the diameter of car or bicycle rims as well as hard drives and storage media:
1 ′ ′ = 25.4 mm
  • lm instead of m (abbreviation for lon end M.eter, outdated, than a sum of individual lengths for prisms with the same cross-section)
  • Std. L instead of L under standard conditions (standard pressure and temperature)
  • Often in connection with American products
  • Colloquially incorrect: folding rule for yardstick


  • qm instead of m² (abbreviation for Quadratmeter, obsolete)


  • ccm instead of cm³ (abbreviation for C.ubiccentimeter, obsolete)
  • cbm instead of m³ (abbreviation for C.ubicmeter, obsolete)


  • Grave instead of kilograms
  • Kilos instead of kilograms
  • Hecto instead of hectogram (especially in Italy)
  • Deka instead of Dekagramm (especially in Austria, Hungary and the Czech Republic)
  • Re-use of the pound and the hundredweight (in the sense of 0.5 kg or 50 kg, the latter in Switzerland, Austria and Russia 100 kg)
  • Weight (according to DIN 1305 equal to mass) instead of weight

Time and time-related quantities

  • superscript characters h, m and s for specifying times in hours, minutes and seconds (from midnight) on a time scale; this notation was used in DIN 1355 time, January 1943 edition, recommended.
  • m instead of min for the time unit minute; This notation was also recommended in DIN 1355 from 1943 on, "if it cannot be confused with m (meter)".
  • kmh instead of km / h (speed unit)
  • Kilometers per hour instead of kilometers per hour for km / h
  • “Auxiliary word / time unit” instead of specifying frequencies in the unit 1 / s or s−1
    • Umin, rpm, rpm, but also rpm, instead of “speed in rpm or min−1
    • flops, but also flop / s (floating-point operations per second)
    • Mips, but also Mi / s (Mega instructions per second), would be correct “number of instructions in 10−6 Seconds "

Electromagnetic quantities

  • Ampere written with an accent in German-speaking countries
  • Incorrect unit symbol "VAr" for the Var, the special name of the unit watt when specifying reactive power; correct is the unit symbol "var".
  • Designation of Siemens as "Mho", occasionally in the Anglo-American language area
  • Voltage instead of better electrical voltage, operating voltage, mains voltage is not a violation of the SI because it does not regulate the naming of physical quantities
  • Electron volts instead of electron volts


  • Shortening from “degrees Celsius” to “degrees”; the degree (°) is a unit of the plane angle and the old CGS unit degree (deg.) for temperature differences.
  • The outdated designation "Degrees Kelvin" for "Kelvin".


  • In the nutrition sector, the calorific value of food is still given in calories (cal) or kilocalories (kcal).


  • The use of horsepower (horsepower) is still more popular, especially in motor vehicles, instead of (kilo) watts. In the SI, however, PS is the unit symbol of the Petasiemens.

Dimensionless sizes

  • It is generally permissible to specify piece (Pieces, pieces) in the sense of a dimensionless quantity for macroscopic quantities.

Category: System of sizes and units